1) Field of the Invention
The present invention relates to an optical device which can be preferably used as a resource in the wavelength division multiplex (WDM) optical communication system.
2) Description of the Related Art
In accordance with a recent abrupt progress in the digital communication, the development of the WDM optical communication system has been strongly required. This WDM optical communication system requires an optical wave converter in order to utilize a limited number of channels in an efficient manner by channel switching. In conventional wavelength converters, XGM type wavelength converters utilizing cross gain modulation and XPM type wavelength converters utilizing cross phase modulation are known.
In the XGM type optical wavelength converter, an intensity-modulated input optical signal having a wavelength xcex1, and an optical signal having a wavelength xcex2 and a constant amplitude are supplied to a semiconductor optical amplifier, and a polarity-inverted output optical signal having a wavelength xcex2, is produced by utilizing a difference in gain for an optical power impinging upon the semiconductor optical amplifier.
The XPM type wavelength converter utilizes the principle of Mach-Zehnder interferometer. In this type of device, an input side of a waveguide upon which an input optical signal having a wavelength xcex2 is divided into two waveguides, a semiconductor optical amplifier is arranged in one of the waveguides, and these two waveguides are set to be in-phase for light having a wavelength xcex2to be modulated. When an input optical signal having a wavelength xcex1 and an optical signal having a wavelength xcex2 and a constant amplitude propagate, there is produced a phase difference of xcfx80/2 between the two waveguides due to the function of the input signal. By utilizing this phase difference, an inverted optical output having a wavelength xcex2 is generated.
Since the known XGM type wavelength converter utilizes the saturated gain of the optical amplifier, the extinction ratio of this optical waveguide converter is small.
In addition, it has inherent drawback that only the inverted optical output signal is produced and non-inverted output signal could not be produced.
In the XPM type optical wavelength converter, although it is possible to obtain a sufficiently large extinction ratio, since it reveals a periodical response, an extremely severer tolerance is required for a device length. Therefore, a through-put of the known XPM type optical wavelength converter is substantially reduced.
Furthermore, the above mentioned known optical wavelength converter is relative large in size. That is to say, the typical size of the known optical converter is not smaller than several to ten millimeters, and thus it is practically difficult to integrate it as a single chip.
It is therefore an object of the present invention to realize a novel and useful optical device which can overcome the aforementioned drawbacks and can have a large extinction ratio.
It is another object of the present invention to provide an optical device which can produce an non-inverted output signal and can operate in the digital manner.
It is still another object of the present invention to provide an optical device which can be manufactured by a relatively simple process and which can operate as a wavelength converter or a waveform shaper.
According to the invention, an optical device for converting an input optical signal into an optically amplified output optical signal comprising:
a semiconductor substrate having mutually opposing first and second surfaces;
a waveguide structure comprising a plurality of semiconductor layers formed on said first surface of the semiconductor substrate and having an incident surface upon which an input optical signal is made incident and an exit surface opposed to said incident surface, said incident and exit surfaces being perpendicular to the semiconductor layers;
a first electrode formed on said second surface of the semiconductor substrate;
a second electrode formed on the top of said waveguide structure such that the second electrode is opposed to said first electrode; and
a DC bias source connected across said first and second electrodes such that carriers are injected into said waveguide structure for amplifying said input optical signal and an amplified output optical signal is emitted from said exit surface;
wherein said semiconductor layers of the waveguide structure are composed of semiconductor materials whose refractive indices vary according to an amount of carriers injected from said first and second electrodes and stored therein;
said first and second electrodes are formed such that a carrier injection region into which carriers are injected through the electrodes and a non-carrier-injection region into which carriers are not substantially injected are formed adjacent to each other in the waveguide structure; and
said waveguide structure is constructed such that, in a carrier injection operation state, when an input optical signal of a first power level propagates through the waveguide structure, a refractive index of the carrier injection region becomes higher than that of the non-carrier-injection region and the carrier injection region constitutes an optical waveguide which guides input light wave from said incident surface to said exit surface, and when an input optical signal of a second power level lower than the first power level propagates through the waveguide structure, a refractive index of the carrier injection region is kept lower than that of the non-carrier-injection regions and the input optical signal is emitted through said non-carrier-injection.
The present invention positively utilizes the optical amplification effect and the free carrier plasma effect in which a refractive index of a semiconductor material is in inverse-proportion to a concentration of carriers injected in the material and stored therein. The first and second electrodes are arranged such that the carrier injection region into which carriers are injected through the electrodes and the non-carrier-injection regions into which carriers are not substantially injected are formed adjacent to each other in the waveguide structure. When mass carriers are injected into the carrier-injection region through the electrodes, these mass carriers are stored in this region and the refractive index of this region is reduced relatively lower than that of the adjacent non-carrier-injection region. However, when the light wave propagates through the waveguide structure, the carriers stored in the carrier-injection region are consumed and the carrier concentration of the waveguide is decreased. The decrease in carrier concentration causes in turn a relative increase in the refractive index of this region higher than that of the adjacent non-carrier-injection regions. In addition, since an amount of consumed carriers substantially corresponds to a power level of the propagating light wave, the propagation of a light wave having a higher power level causes a further decrease in the carrier concentration of the carrier-injection region and thus the refractive index of this region to be significantly higher than that of the adjacent non-carrier-injection regions, thereby further enhancing the optical confinement effect. This results in that there is formed in the waveguide structure a propagation path for the optical signal with a higher refractive index than that of the surrounding medium. Once such a propagation path is formed, the optical confinement effect and the induced emission generated in the waveguide are further enhanced, which induces a state of positive feedback. Therefore, this type of waveguide is referred as an xe2x80x9coptically induced waveguidexe2x80x9d.
Conversely, when a signal light having a low power level propagates through the waveguide structure, since the amount of the carriers consumed by the optical amplifying function is rather small, a mass of carriers are still remained in this region, with the refractive index of this region being kept low. As the refractive index of this region is relatively lower than that of the adjacent non-carrier-injection regions, the signal light having a low power level is absorbed by the adjacent region having a relatively high refractive index and is radiated through the region.
The present invention is based on the aforementioned understanding. According to the invention, the waveguide is switched according to the power level of the input optical signal between a guiding mode in which the input optical signal is guided through the waveguide to the exit surface and an anti-guiding mode in which the input optical signal is emitted through the non-carrier-injection regions. Utilizing this mode switching, a wavelength converter and a waveform shaper can be realized with a simple manufacturing process.
According to the present invention, a nonlinear input/output characteristic can be obtained by utilizing the free carrier plasma effect and the optical amplification effect. That is, when the power level of the input signal light is low, almost all input light is absorbed by the non-carrier-injection region and consequently the amount of the carriers consumed by the optically amplifying function is very small. Although the increase of the power level of the input optical signal causes the amount of the carrier consumption in the waveguide to be increased, the anti-guiding mode is still maintained as long as the refractive index of the waveguide remains lower than the refractive index of adjacent non-carrier-injection regions, allowing only a little signal light to be transmitted up to the exit surface. When the refractive index of the waveguide becomes to substantially equal to that of the non-carrier-injection regions because of the increased carrier consumption, the guiding effect is abruptly generated such that the input optical signal can propagate through the waveguide to the exit surface. This propagation of the signal light of a higher power level abruptly enhances the optical amplification effect to emit the optically amplified optical signal. As a result, a nonlinear input/output characteristic with a threshold value can be obtained, which could never be obtained by conventional semiconductor optical amplifiers. Owing to this nonlinear input/output characteristic, the optical device of the invention can act in a digital manner.
More important fact is that the optical device of the present invention outputs xe2x80x9chighxe2x80x9d signal light for xe2x80x9chighxe2x80x9d input signal light, and also outputs xe2x80x9clowxe2x80x9d signal light for xe2x80x9clowxe2x80x9d input signal light. This results in that a non-inverted output optical signal can be produced.
By utilizing the nonlinear input/output characteristic of the present invention, the optical devices with various functions can be realized. For example, by supplying an optical signal having a wavelength xcex1, together with a continuous wave having a wavelength xcex2, an optically amplified signal light having a wavelength xcex2 and modulated according to the signal light having the wavelength xcex1 can be output. This realizes a wavelength converter having a function of optical amplifier with a simple construction.
Also, by supplying a weak optical signal having a wavelength xcex1 and a continuous wave having the same wavelength, the output optical signal which is optically amplified and has a high S/N ratio can be obtained. In contrast, conventional semiconductor optical amplifiers cannot obtain an output optical signal having a high S/N ratio, because xe2x80x9chighxe2x80x9d signal components and xe2x80x9clowxe2x80x9d signal components in the input optical signal are both equally amplified, that is, noise components are equally amplified. In this manner, the present invention has a special functional effect which cannot be achieved by conventional semiconductor optical amplifiers.